Magnetic core structures for instrument transformers

ABSTRACT

A magnetic core having a plurality of lamination layers. Gaps are located in the lamination layers to reduce residual magnetic flux and are displaced from each other in adjacent lamination layers to form a stepped gap pattern across the magnetic core. A sheet of solid insulating material is disposed in the gaps formed by the lamination layers to maintain the length of the gaps.

United States Patent 1191 Wentz et al. v

1451 Nov. 27, 1973 MAGNETIC CORE STRUCTURES FOR INSTRUMENT TRANSFORMERS [75] Inventors: Edward C. Wentz, Sharon; Belvin B. I

Ellis, Pulaski; Angelo A. Delauren- -tis, Sharpsville, all of Pa.

[73] Assignee: Westinghouse Electric Corporation, Pittsburgh, Pa.

22 Filed: Sept. 1, 1972 21 App1.'No.: 285,696

[52] US. Cl 336/178, 336/217, 336/219 [51] Int. Cl. H01r 27/24 [58] Field of Search... 336/219, 217, 216,

[56] References Cited UNITED STATES PATENTS 9/1969 Olsen 336/219 X 8/1960 Fredrickson.. 336/219 X l/1971 Jones et a1. 336/219 X 3,179,634 4/1965 Edwards 174/110 N 2,523,071 9/1950 Somerville 336/217 X 3,339,163 8/1967 Wentz et a1..- 336/217 X FOREIGN PATENTS OR APPLICATIONS 924,543 8/1947 France 336/219 Primary Examiner-Thomas J. Kozma Attorney-A. T. Stratton et a1.

[57] ABSTRACT A magnetic core having a plurality of lamination layers. Gaps are located in the lamination layers to reduce residual magnetic flux and are displaced from each other in adjacent lamination layers to form a stepped gap pattern across the magnetic core. A sheet of solid insulating material is disposed in the gaps formed by the lamination layers to maintain the length of the gaps.

5 Claims, 5 Drawing Figures Bjorklund 336/219 X using current transformers s is, a d r ay W t the dven iif fainte h h'exhi iia' re vel 19 MAGNETIC CORE STRUCTURES oR INSTRUMENT T ANsro z nns BACKGROUND or THE INVEN ION 1. Field of the lnvention This invention relates, in general, to electrical inductive apparatusand, more specifically, to magnetic cores for instrument transformers. i i

2. Description of the Prior Art Instrument transfer: rs are used to monitor voltages and currents existing power transmission and distribution systems. Currenvtyne instrument transformers, or current transformers, usually include a magnetic core structure having a winding disposed thereon. The core and winding assembly are suitably placed in the magnetic field created by the current'to he measured. A change in the measured current produces a change in the surrounding flux, thus the volt e induced the winding of the current transformer'is changed.

Current transformers which are placed around electrical bushings generally haye a closed a of circular shape. Brior art currenttran structed in this mannerhave a substa v v l magnetic core structure does contain air gaps. Also, prior art cores used current transformers contain no joint at all since it is practical wind the windingaround a closed core. l l '1 i The performance of current transformers is critical when used in control systems w ate or formance to be adequate, the current a vided by the current transformer be suffic nt to electricalpower between circuits. ln'orderlfor the perproperly control the associated co t Current transformers are used in co nection with a variety of functions, including metering andrelaying.

h Output m th ss rsn 'tiss ciently representative of the measured current to provide the proper metering or rela The current ein measured rnay e interrupted at any point during the polarityc'yclej therefore, possibility of creating a residual flux the magnetic core of the current transformer is'great. en using conventional core materials and construction arrangements the residual flux maybe as highas Q0 percent of the peak flux during normal operation.

dua flux a P e n or r inserted 99 of the components and circuits connectedtothecurrent transformer. When the current being measured begins to flow after a period during m w it I flow, the flux provided by thetlowing current is added vectorially to the residual The e k is an incora Cu r r n form was l the d anesidual fluxes are in phase, saturation of magnetic core may occur.

The residual flux is reduced to a nominal yalue after several y e 6f a er t n flu hat hes. into the core. This partly accounts for feasibility of high i u flu comp nent ponents.

vsntdigb i n th e a w h m b Several methods may be used to reduce the residual flux in current transformer cores. Selection of the magnetic material is important and magnetic steels are available which yieldrelatively low residual flux components. However, such materials are costly and their .use is undesirable from an economic standpoint. Therefore, it is desirable, andit is an object of this invention, to provide a current transformer which exhibits relatively low residual flux components with the use of standard grade magnetic steel materials.

The theory on the use of air gaps in magnetic cores to the residual tlux components is known by those skilled in the" art. A magnetization curve for a magnetic core illustrates the magnetic flux density B respect to the magnetic field intensity H. The

niaximumamount of residual flux is depicted graphically as the amount of flux density which exists when the field intensity is zero. This occurs where the magne tization curve crosses the ordinate corresponding to zero field intensity. By introducing an air gap into the magnetic core, the magnetization curve is shifted and intercepts the zero field intensity ordinate at a lower point than the curve without an air gap. Unfortunately, reducing residual flux by constructing an air gap in the magnetic core increases the fieldintensi ty required to produce a given amount of flux densityfTo keep the magnetic characteristics of the magnetic core within usable limits, the effective air gap must be kept as small as possible while still providing a signif cant reduction in the maximum residual fl Conventional air gap arrangements and construction technqiues exhibit certain charactersitics which in ake the use of conventional air gaps in current transfornier cores unsatisfactory. air gap cut straight across the magnetic core has the same gap surface area as the cross-sectional area of the magnetic core. To keep the flux density and the field density relationship within satisfactory limits in current transformer "coresf the length of the gap must be extremely small. Nonmagnetic material placed in the gap region to maintain its length would be so thin that material now available would not he physically strong enough to resist the mechanical forces subjected upon the gap during the life of theIcurrent transformer. Therefore, it is desiralile, and it is an object of this invention, to provide a current transformer core wherein the effective gap is less than the actual gap, thereby allowing readily available materials to'he inserted therein to maintain the gap length. The length of an air gap which is cut straight across a magnetic core is difficult to keep constant becauseof diff culty in bracing the core in the gap regionQ'Under 'high nur conditions, such a gap tends to i become wedge-shaped, thereby changing the effective gap distance and the characteristics of the magnetic core. A gap positioned straight across a magnetic core causes a certain amount of losses due to unavoidable burrs on the ends of the lamination layers of the magnetic core which are produced during the cutting'of gap. In th ra ht a q ss i th :Ibfirrs are aligned mmetr cally about the longitudinal axis of the core; that is, across the flat face of the gap. Burrs from one lamination layer contact adjacent lamination layers arid an eddy u r nt Pat a ro s th f f t e sap b .e tablished. Therefore, it is desirable, and'it'is an oh'ect f this n ntiq' t Pr id a c te ansfer ar wherei a ap h in ma be u t y to reduce eddy current losses occasioned by the alignment of burrs on the end of the core lamination layers. SUMMARY OF THE INVENTION There is disclosed herein a new and useful magnetic core structure for current-type instrument transformers. The core includes a plurality of lamination layers "maintain the length of the gaps, The spacer 42 is concontaining gaps between the ends thereof. Adjacent lamination layers are displaced in a predetermined pattern to provide a stepped gap pattern across the magnetic core. A solid insulating material is placed in the gaps to maintain the spacing or length thereof. By stepping the gaps, the gaps are positioned diagonally across the magnetic core. This increases the gap area and effectively allows the length of the gap to be increased to a practical value. Stepping the gaps also distributes the burred ends of the lamination layers, and thus reduces eddy currents at the face of the gap.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENTS Throughout the following description, similar reference characters refer to similar members in all figures of the drawing.

Referring now to the drawing, and FIG. 1 in particular, there is shown an electrical bushing with a bushing-type current transformer 12 disposed thereon. The bushing includes the expansion cap 14, the upper insulator 16, the lower insulator l8, and the conductor tube structed of asolid insulating material such as cellulosic kraft paper, wholly aromatic polyamide paper, poly-' imide film, or anyother suitable material. Gap'44 is created by the separation of the.ends 46 and 48 of the lamination layer 50. The adjacent gap 52, which is formed by the adjacent lamination layer 54, is transversely displaced from the gap 44. The amount of' gap displacement depends on the thickness of the spacer 42, the thickness of the lamination layers, and the distance adjacent lamination layers are stepped. It has been determined that a practical core may be constructed when the gap displacement is approximately six times the lamination thickness. I

The thickness of steel lamination layers is generally about 0.011 inch to 0.102 inch. The thickness of the spacer may vary depending on the size and rating of the current transformer. Typical values range between 0.001 inch and 0.100 inch. The gap length may be changed conveniently by changing the thickness of the spacer 42.0r by placing a plurality, of spacers 42 in the gap region 33.

A transverse gap displacement in adjacent laminations equal to six times the thickness of a lamination layer provides a practical compromise between satisfactory core performance and convenient core construction. The relative position of the end 48 with respect to the end 56 depends on the manner in which the core is constructed, and they may be displaced from each other differently than shown in FIG. 2A without departing from the spirit of the invention.

20. The bushing 10 is attached to the enclosure 22 of the electrical apparatus on which the bushing is mounted, such as a power transformer.

The current transformer 12 is positioned around the lower insulator 18. The current transformer 12 includes a laminated magnetic core 24, a winding structure 26 disposed thereon, and solid insulation 28 disposed around the core and winding assembly. The leads 30 provide means for transferring an electrical output to associated metering or relaying apparatus.

The magnetic core 24 is constructed of layers of laminations which are disposed concentrically about each other. FIG. 2 is a view of the magnetic core 24 illustrating novel features of its construction. An inner lamination layer 32 contains a gap at location 34. An outer lamination layer 36 contains a gap at location 38. Intermediate lamination layers 40 are positioned between the inner lamination layer 32 and the outer lamination layer 36. Gaps in adjacent lamination layers are displaced to form a stepped gap pattern across the magnetic core 24. It is within the contemplation of this invention that the general shape of the magnetic core 24 may be other than circular, such as square or rectangu- Generally, the dimensions of the spacer 42 are sufficient to provide spacing material located throughout the longitudinal distance of the joint which is perpendicular to the plane of FIG. 2A. The thickness of the spacer 42 is an important factor in governingthe shape of the spacer 42 in a complete core. A relatively thick spacer 42 would maintain a substantially straight shape when moderate pressures are applied by the bands 31. A relatively thin spacer 42 would conform to the stepped shape formed by the lamination layer ends.

FIG. 28 illustrates an embodiment of this invention wherein adjacent lamination layers overlap for a short distance. The spacer 42 is curved to conform somewhat to the lamination layer step's. With a, thin flexible spacer 42-,,the spacer 42 may fill the gaps completely when-the stepping distance is reduced to the thickness of the spacer 42. 3 v

FIG. 3 illustrates -the upper portion of magnetization, or B-H, curves for magnetic cores. Curve 60 is representative of a magnetic core without a gap. When the field intensity decreases to zero, and remains at zero, the residual flux B, in the ungapped core may be as high as percent of the peak flux.

Curve 62 is representative of a magnetic core containing a gap. The effect of a gap is to tilt the B-H curve of the core. This reduces the'residual flux B in the magnetic core. From curve 62, it can be seen that when the field intensity decreases to zero, the residual flux B is much lower than that provided by the ungapped core represented by curve 60. A suitable and workable value for B is percent of the peak flux density.

The novel magnetic core structure taught by this invention increases the overall core gap area, thus allowing an increase in gap length or spacing without changing the effective gap length compared to a gap cut straight across the core. This permits satisfactory placement of standard materials in the gap region to maintain the length of the gap. Burrs produced during the cutting of the lamination layers are displaced from the burrs of adjacent lamination layers, thus eddy currents due to lamination burrs are reduced. The solid insulating material contained within the gap allows the core to be clamped and banded tightly together while still maintaining the desired gap length.

Since numerous changes may. be made in the abovedescribed apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all of the matter contained in the foregoing description, or shown in the accompanying drawings, shall be interpreted as illustrative rather than limiting.

We claim as our invention:

1. A magnetic ,core for electrical inductive apparatus, comprising an inner lamination layer having a gap therein, an outer lamination layer having a gap therein, said outer lamination layer being concentrically located around said inner lamination layer, intermediate lamination layers concentrically located between said inner and outer lamination layers with each intermediate lamination layer having a gap therein, the gaps in adjacent lamination layers being transversely displaced from each other to provide a stepped gap pattern which progresses radially across said magnetic core, and a sheet of solid insulating material disposed in said gaps to. maintain the length of said gaps.

2. The magnetic core of claim 1 wherein the solid insulating material comprises a sheet of cellulosic paper.

3. The magnetic core of claim 1 wherein the solid insulating material comprises a sheet of wholly aromatic polyamide paper.

4, The magnetic core of claim 1' wherein the solid insulating material comprises polyimide film.

5. A magnetic core for .a current-type instrument transformer, comprising a plurality of concentrically located cylindrical lamination layers, each of said lamination layers containing a gap, the location of gaps in adjacent lamination layers being transversely displaced from each other to provide a stepped gap pattern which progresses radially across said magnetic core a sheet of solid insulating material disposed in said gaps to maintain the length of said gaps, and means for tightly pressing the ends of said lamination layers against said sheet of solid insulating material. 

1. A magnetic core for electrical inductive apparAtus, comprising an inner lamination layer having a gap therein, an outer lamination layer having a gap therein, said outer lamination layer being concentrically located around said inner lamination layer, intermediate lamination layers concentrically located between said inner and outer lamination layers with each intermediate lamination layer having a gap therein, the gaps in adjacent lamination layers being transversely displaced from each other to provide a stepped gap pattern which progresses radially across said magnetic core, and a sheet of solid insulating material disposed in said gaps to maintain the length of said gaps.
 2. The magnetic core of claim 1 wherein the solid insulating material comprises a sheet of cellulosic paper.
 3. The magnetic core of claim 1 wherein the solid insulating material comprises a sheet of wholly aromatic polyamide paper.
 4. The magnetic core of claim 1 wherein the solid insulating material comprises polyimide film.
 5. A magnetic core for a current-type instrument transformer, comprising a plurality of concentrically located cylindrical lamination layers, each of said lamination layers containing a gap, the location of gaps in adjacent lamination layers being transversely displaced from each other to provide a stepped gap pattern which progresses radially across said magnetic core, a sheet of solid insulating material disposed in said gaps to maintain the length of said gaps, and means for tightly pressing the ends of said lamination layers against said sheet of solid insulating material. 